1. Field of the Invention
The present invention relates to low-alloy heat-resistant steels which exhibit excellent performance as large turbine rotor members, heat resistant components for generating plants, and components for devices which are subjected to high temperatures, and relates to heat treatment methods for the low-alloy heat-resistant steels and turbine rotors comprising the low-alloy heat-resistant steels.
2. Description of the Related Art
Conventionally, as heat resistant steels for high temperature turbine rotor members for use in steam turbine plants for thermoelectric power generation, 12Cr steels, which belong to high-Cr steels (see Japanese Patent Applications, First Publications (Kokai), Nos. Sho 60-165359, and Sho 62-103345) and CrMoV steels, which belong to low-alloys (see Japanese Patent Application, First Publication (Kokai), No. Sho 60-70125), have been exclusively used. 12Cr steels have superior high temperature strength and can hence be used in plants having a steam temperature up to 600xc2x0 C. However, 12Cr steels are disadvantageous in that the production of the material is difficult and costly. Of these, the use of CrMoV steel is restricted to plants having a steam temperature up to 566xc2x0 C. because of its limited high temperature strength. Moreover, cooling of the rotor may be required depending on the steam temperature, which is disadvantageous in that it complicates the plant.
However, in recent years, further improvement in energy efficiency has been desired, and if it is desired to raise the operational temperature of a steam turbine, a steel of a conventional type is insufficient in mechanical properties at high temperatures, particularly in terms of creep strength. Accordingly, the demand to develop a material which is durable in use at higher steam temperatures has been growing. Conventionally, a CrMoV steel is used after quenching the CrMoV steel heated to a temperature of about 950xc2x0 C. A higher heating temperature before quenching results in a higher strength of the material because precipitation of a pro-eutectoid ferrite phase, which is soft, is inhibited, and dissolution of the strengthening elements in a solid solution is promoted. However, another problem arises in that a higher heating temperature before quenching causes creep embrittlement of the material. Therefore, the heating temperature before quenching cannot be raised. Although attempts have been made in which elements such as cobalt, niobium, and tantalum, were additionally used in order to inhibit the precipitation of a pro-eutectoid ferrite phase, a satisfactory material has not yet been obtained.
Accordingly, an object of the present invention is to provide a low-alloy heat-resistant steel in which when it is used to manufacturing a large element, which have uniform superior high temperature properties through a surface layer to a center part. In particular, an object of the present invention is to provide a low-alloy heat-resistant steel which has high creep embrittlement resistance.
Another object of the present invention is to provide a heat treatment method for preparing the novel low-alloy heat-resistant steels.
Another object of the present invention is to provide a turbine rotor comprising the novel heat resistant steel.
In order to achieve the above objects, the present inventors have diligently carried out research and have discovered that the sizes of crystal grains comprising the matrix greatly affects the properties of a steel at high temperatures, particularly the creep embrittlement resistance. That is, the following was discovered. When large elements, such as turbine rotors are cooled in a conventional method, in a center part thereof, since a suitable amount of a pro-eutectaid ferrite phase is easily precipitated, the crystal grains are relatively fine, a high toughness can be obtained, and the creep embrittlement does not occur. In contrast, in the surface layer thereof, since a pro-eutectaid ferrite phase is hard to be precipitated and the crystal grain easily becomes large, the Charpy impact absorbed energy decreases. Thereby, there is the possibility that the material in the surface layer is embrittled and that creep embrittlement occurs.
Consequently, it was also found that the low-alloy heat-resistant steels can be obtained, which have high toughness, excellent high temperatures properties, in particular, in which the creep embrittlement does not occur, and which are suitable for large elements, such as turbine rotors, not only by mixing alloy components in predetermined proportions and minimizing the amount of minor impurity elements which are harmful, but also by adjusting the crystal grain size of the matrix.
First, the measuring method for crystal grain size will be described below. As a measuring method for crystal grain size, Japanese Industrial Standard JIS G 0551 (1998) defines an austenitic grain size determination for steel and JIS G 0552 (1998) defines a ferritic grain size determination for steel. The low-alloy heat-resistant steels of the present invention comprise a metallic structure containing a ferrite phase and a bainite phase which is prepared by quenching from high temperatures, such as an austenitic phase stabilized temperature range, and thereby a pro-eutectoid ferrite phase is precipitated. Therefore, in the present invention, crystal grain size of the composite containing a ferritic phase and a bainite phase is specified. Specifically, in the present invention, the boundary between a bainite phase and a pro-eutectoid ferrite phase, the boundary between pro-eutectoid ferrite phases, and the boundary between austenitic particles which are to be transformed into a bainite phase are defined as the crystal grain boundary, and the size of the area surrounded by the crystal grain boundary is defined as the crystal grain size.
In the present invention, the ferritic grain size determination for steel comprising mixed crystal grains, which is defined by JIS G 0552 (1998), is adopted. In other words, the measuring method is one, in which a photograph of crystal grains which appears at a corroded surface of a test piece is taken using a microscope, and the sizes of the crystal grains are measured by a cross segment method. Moreover, when the crystal grain number is lower, the crystal grain size is larger.
Next, of the high temperature properties, the creep rupture strength of a creep test on a notched test piece (abbreviated as xe2x80x9cnotched creep testxe2x80x9d below) will be described. Since a turbine rotor is subjected to high temperatures for a long time under stress during operation, deterioration in the strength of the material with age is a concern. The quality of turbine rotor members has been hitherto evaluated only by high temperature unnotched creep tests, as defined by the Japanese Industrial Standards or the like. However, the present inventors have discovered a method of evaluating high temperature strength properties of the material, particularly the creep embrittlement resistance, in a high temperature creep test on a notched test piece.
When a stress is applied to a steel product at a high temperature, even if the stress is relatively small, the steel product plastically deforms very gradually to become elongated, and finally the elongation proceeds, rapidly narrowing a part of the steel product, which results in rupture of the steel product. This phenomenon is called xe2x80x9cthe creepxe2x80x9d or xe2x80x9cthe creep rupture phenomenonxe2x80x9d. In addition, the cross area ratio between the test piece and the ruptured test piece is called xe2x80x9cthe creep rupture ratioxe2x80x9d. In a high temperature creep test, a constant static load is applied to a material for a long time at a high temperature, and the time elapsed before rupture is measured. As a test piece, a round bar having a constant cross section is used. The measuring method is defined by JIS Z-2271. The measuring methods defined by the JIS standards are for creep tests on unnotched test pieces, and test pieces which are finished by smoothly shaving between gauge marks in the portion to be measured are used in these methods.
In contrast, in a creep test on a notched test piece (abbreviated as xe2x80x9cnotched creep testxe2x80x9d below), a test piece having a notch between gauge marks is used. The cross section of the portion (the cross section of the bottom of the notch) to be stretched and subject to measurement is set to be the same as the cross section of the part subject to the measurement in a unnotched creep test, and the stress is determined. In an unnotched creep test, a tensile stress which is applied gradually increases the distance between gauge marks, and narrows the portion between the gauge marks, which finally will rupture. In contrast, if a notch is formed in a test piece, a stress which counteracts deformation of the notched portion is produced such that the stress surrounds the notched portion (this stress is a so-called xe2x80x9cmultiaxial stressxe2x80x9d), and the test piece finally ruptures without being elongated. In general, with a highly ductile material, the lapse of time before rupture tends to be long because deformation is restricted by the notch. However, depending on the type of steel, embrittlement of some materials gradually proceeds during creep tests, and creep rupture may occur in such a material without deformation (by occurrence of voids or by formation of cracks from connected voids). In this case, a notched test piece ruptures in a shorter time than an unnotched test piece due to the concentrated stress. Such a phenomenon is called xe2x80x9cnotch softeningxe2x80x9d, which can be used as an index for expressing creep embrittlement. That is to say, by conducting creep rupture tests on an unnotched test piece and a notched test piece under the same conditions such as stress and temperature, and comparing the times elapsed before creep rupture, the level of creep embrittlement can be clearly demonstrated. The ratio between the times elapsed before creep rupture in the unnotched creep rupture test and the notched creep rupture test is defined as a creep rupture time ratio.
That is, when the time elapsed before creep rupture in the unnotched creep rupture test is defined as A and that in the notched creep rupture test is defined as B, the creep rupture time ratio is shown by the formula (1).
Creep rupture time ratio=B/Axe2x80x83xe2x80x83(1)
In order to clarify the relationship between the crystal grain size of the matrix and the creep embrittlement, the following tests were carried out.
The crystal grain size was varied by using a test material No. 1, which is in the following Table 1 and which comprises 0.26% by weight of C (in the following, xe2x80x9cby weightxe2x80x9d is omitted), 0.05% of Si, 0.09% of Mn, 0.08% of Ni, 1.46% of Cr, 0.54% of Mo, 2.40% of W, 0.25% of V, 0.006% of P, 0.001% of S, 0.03% of Cu, 0.003% of Al, 0.006% of As, 0.005% of Sn, and 0.0012% of Sb, and changing a forgoing degree and varying the pre-heat treatment. Then, the test materials were heated to 1050xc2x0 C. and were subjected to an oil-hardening which simulated a cooling treatment in which the center part and the surface layer far from the surface at 100 mm of the rotor having a drum diameter of 1,200 mm were cooled. After that, the test pieces for characteristics test were obtained by adjusting the tempering temperature so that the strength at an early age (0.2% yield strength) is in a range of from 588 to 647 MPa.
The structure of the test pieces was observed by an optical microscope, and an austenitic grain size number and an amount of a pro-eutectoid ferrite phase were measured. The austenitic grain size number was measured based on JIS G 0551. The results are shown in the following Table 2. In addition, the Charpy impact absorbed energy was measured. Furthermore, a creep test was carried out. In the creep test, creep rupture time at 600xc2x0 C. and 147 MPa was measured using an unnotched test piece and a notched test piece. The results are shown in the following Table 3.
It is clear from Tables 2 and 3 that an austenitic grain size number of the test pieces Nos. 1xe2x80x941 to 1-4 is 3.2 or 4.1 and a pro-eutectoid ferrite phase is not precipitated in these test pieces, and Charpy impact absorbed energy thereof is 41 (J) or greater. It is also clear that the creep rupture ratio thereof in the unnotched creep test is 72% or greater, and that these test pieces have high toughness. Moreover, in the notched creep test, the rupture does not occur in these test pieces after 14,000 hours. Therefore, it was confirmed that these test pieces have excellent creep embrittlement properties.
In contrast, the test pieces Nos. 1-5 and 1-6 have small austenitic grain size numbers being 2.3 and 2.5, that is, they have large crystal grains, the Charpy impact absorbed energy thereof is 30 J or less, and the creep rupture ratio in the unnotched creep test is 30.3% or less. Therefore, it is confirmed that the test pieces are embrittled. In addition, the lapse of time before rupture in the notched creep test of the test pieces is 7,500 hours or more, this is longer than that of the test pieces of the present invention. However, in the notched creep test, these test pieces did rupture after 10,000 hours. Therefore, it was confirmed that these test pieces have inferior anti-creep embrittlement properties.
Basically, the materials of the present invention, which have excellent anti-creep embrittlement properties, have creep rupture time ratio (notched creep rupture time/unnotched creep rupture time) is 1.97 or greater. In contrast, the comparative materials, which have inferior anti-creep embrittlement properties, have a creep rupture time ratio of 1.39 or less.
From these results, it is clear that when the crystal grain is large, that is, when the grain size number is small, the Charpy impact absorbed energy decreases, and creep embrittlement is significant. Therefore, it is clear that in order to obtain low-alloy heat-resistant steels having excellent high temperature properties, it is necessary to adjust the crystal grain size such that the grain size number of the bainite phase which is transformed from an austenitic phase is 3.0 or greater.
For example, as materials comprising turbine rotors, materials are suitable which have a 0.2% yield strength of 588 MPa or greater, a Charpy impact absorbed energy at room temperature of 9.8 J or greater, a creep rupture time in the unnotched creep test at 600xc2x0 C. and 147 MPa of 3,000 hours or greater, a creep rupture time in the notched creep test at 600xc2x0 C. and 147 MPa of 10,000 hours or greater, a creep rupture time ratio of 1.6 or greater, and an unnotched creep rupture ratio of 50% or greater.
In addition, the manufacturing method for low-alloy heat-resistant steels which are used preferably for large elements, such as turbine rotors, and have the above-mentioned suitable crystal grain size in the center part and the surface layer, was examined. As a result, it was clear that the adjustment of the crystal grain size is effectively carried out by precipitating a suitable amount of the pro-eutectoid ferrite phase and obtaining a composite structure containing the pro-eutectoid ferrite phase and the bainite phase. However, in large elements, since the cooling rate is high in the surface layer and that in the center part is low, if a suitable amount of the pro-eutectoid ferrite phase is made to precipitate in the vicinity of the surface layer, a problem occurs in the center part in that a large amount of the pro-eutectoid ferrite phase, which is not necessary, is precipitated, and the toughness of the element decreases.
The problem can be solved by the first heat treatment method in which a material is heated to high temperatures, such as an austenitize temperature range; after the material is rapidly cooled and quenched, the cooling rate decreases once at a certain temperature, and thereby the temperature difference between in the center part and in the surface layer decreases, that is, the center part is cooled slowly, and thereby a suitable amount of the pro-eutectoid ferrite phase is made to precipitate in the center part; after that, the material is rapidly cooled again. According to the first heat treatment method, a metallic structure, in which the average crystal grain size in the surface layer substantially equals that in the center part, can be obtained.
In addition, the problem can also be solved by the second heat treatment method in which a material is heated to high temperatures, such as into an austenitize temperature range; the material is cooled with a relatively low cooling rate over high temperatures, and thereby a suitable amount of the pro-eutectoid ferrite phase is made to precipitate in the surface layer; after that the material is cooled at a relatively high cooling rate over low temperatures, and thereby the toughness of the material is increased. According to the second heat treatment method, a metallic structure, in which the size of the crystal grain and the pro-eutectoid ferrite phase in the surface layer substantially equals those in the center part, can be obtained.
The metallic structure of the materials which are heat treated in the first and second heat treatment methods comprise the bainite phase structure in which the pro-eutectoid ferrite phase is precipitated. In the pro-eutectoid ferrite phase, carbonitrides of the matrix reinforcing elements, such as Mo, W, V, and the like, are finely dispersed and precipitated by tempering. The pro-eutectoid ferrite phase has been believed to be soft and it was believed that it makes the toughness of the material decrease, and the pro-eutectoid ferrite phase should not be precipitated as much as possible. In contrast, in the present invention, after the pro-eutectoid ferrite phase is reinforced by using the matrix reinforcing elements and a suitable amount of the pro-eutectoid ferrite phase is precipitated, the pro-eutectoid ferrite phase is used effectively to reduce the sizes of the crystal grains.
The turbine rotors comprising such low-alloy heat-resistant steels of the present invention, which are manufactured by such heat treatment methods, comprise a metallic structure in which crystal grain sizes in the surface layer substantially equal those in the center part, and the crystal grain size is most suitable, and the turbine rotors have high strength and toughness at low temperatures and have high temperature properties. Therefore, the turbine rotors do not experience creep embrittlement. In addition, since large elements can also be simply heat treated, the manufacturing cost can be reduced, and the manufacturing time can also be shortened.
In other words, in order to achieve the objects, the present invention provides a first low-alloy heat-resistant steel comprising:
carbon in an amount of 0.20 to 0.35%,
silicon in an amount of 0.005 to 0.35%,
manganese in an amount of 0.05 to 1.0%,
nickel in an amount of 0.05 to 0.3%,
chromium in an amount of 0.8 to 2.5%,
molybdenum in an amount of 0.1 to 1.5%,
tungsten in an amount of 0.1 to 2.5%,
vanadium in an amount of 0.05 to 0.3%,
phosphorus in an amount not greater than 0.012%,
sulfur in an amount not greater than 0.005%,
copper in an amount not greater than 0.10%,
aluminum in an amount not greater than 0.01%,
arsenic in an amount not greater than 0.01%,
tin in an amount not greater than 0.01%,
antimony in an amount not greater than 0.003%, and
the balance being iron and unavoidable impurities, and containing a metallic structure having an austenitic grain size number in a range of from 3 to 6.
According to the first low-alloy heat-resistant steel, the creep properties are improved by adding tungsten into conventional CrMoV steels. In addition, the creep properties, in particular, the creep embrittlement resistance, is improved by minimizing the permissible amount of minor impurity elements, such as P, S, Cu, Al, As, Ti, Sb, and the like, which are harmful in causing creep embrittlement, and adjusting an austenitic grain size number of the crystal grain into a range of from 3 to 6.
In order to achieve the objects, the present invention provides a second low-alloy heat-resistant steel comprising:
carbon in an amount of 0.20 to 0.35%,
silicon in an amount of 0.005 to 0.35%,
manganese in an amount of 0.05 to 1.0%,
nickel in an amount of 0.05 to 0.3%,
chromium in an amount of 0.8 to 2.5%,
molybdenum in an amount of 0.05 to 1.5%,
tungsten in an amount of 0.01 to 2.5%,
vanadium in an amount of 0.05 to 0.3%,
cobalt in an amount of 0.1 to 3.5%,
phosphorus in an amount not greater than 0.012%,
sulfur in an amount not greater than 0.005%,
copper in an amount not greater than 0.10%,
aluminum in an amount not greater than 0.01%,
arsenic in an amount not greater than 0.01%,
tin in an amount not greater than 0.01%,
antimony in an amount not greater than 0.003%, and
the balance being iron and unavoidable impurities, and containing a metallic structure having an austenitic grain size number in a range of from 3 to 6.
In the second low-alloy heat-resistant steel, cobalt is added into the first low-alloy heat-resistant steel. Thereby, the toughness thereof is more improved. In addition, similar to the first low-alloy heat-resistant steel, the creep properties, in particular, the creep embrittlement resistance is improved by minimizing the permissible amount of minor impurity elements, such as P, S, Cu, Al, As, Ti, Sb, and the like, which are harmful in causing creep embrittlement, and adjusting an austenitic grain size number of the crystal grain into a range of from 3 to 6.
In addition, in order to achieve the objects, the present invention provides a third low-alloy heat-resistant steel comprising:
carbon in an amount of 0.20 to 0.35%,
silicon in an amount of 0.005 to 0.35%,
manganese in an amount of 0.05 to 1.0%,
nickel in an amount of 0.05 to 0.3%,
chromium in an amount of 0.8 to 2.5%,
molybdenum in an amount of 0.1 to 1.5%,
tungsten in an amount of 0.1 to 2.5%,
vanadium in an amount of 0.05 to 0.3%,
at least one of niobium in an amount of 0.01 to 0.15%, tantalum in an amount of 0.01 to 0.15%, nitrogen in an amount of 0.001 to 0.05%, and boron in an amount of 0.001 to 0.015%,
phosphorus in an amount not greater than 0.012%,
sulfur in an amount not greater than 0.005%,
copper in an amount not greater than 0.10%,
aluminum in an amount not greater than 0.01%,
arsenic in an amount not greater than 0.01%,
tin in an amount not greater than 0.01%,
antimony in an amount not greater than 0.003%, and
the balance being iron and unavoidable impurities, and containing a metallic structure having an austenitic grain size number in a range of from 3 to 6.
In the third low-alloy heat-resistant steel, at least one of niobium, tantalum, nitrogen, and boron are added into the first low-alloy heat-resistant steel. Thereby, the unnotched creep properties thereof are further improved. In addition, similar to the first low-alloy heat-resistant steel, the creep properties, in particular, the creep embrittlement resistance, is improved by minimizing a permissible amount of minor impurity elements, such as P, S, Cu, Al, As, Ti, Sb, and the like, which are harmful in causing creep embrittlement, and adjusting an austenitic grain size number of the crystal grain into a range of from 3 to 6.
In addition, in order to achieve the objects, the present invention provides a fourth low-alloy heat-resistant steel comprising:
carbon in an amount of 0.20 to 0.35%,
silicon in an amount of 0.005 to 0.35%,
manganese in an amount of 0.05 to 1.0%,
nickel in an amount of 0.05 to 0.3%,
chromium in an amount of 0.8 to 2.5%,
molybdenum in an amount of 0.1 to 1.5%,
tungsten in an amount of 0.1 to 2.5%,
vanadium in an amount of 0.05 to 0.3%,
cobalt in an amount of 0.1 to 0.3%,
at least one of niobium in an amount of 0.01 to 0.15%, tantalum in an amount of 0.01 to 0.15%, nitrogen in an amount of 0.001 to 0.05%, and boron in an amount of 0.001 to 0.015%,
phosphorus in an amount not greater than 0.012%,
sulfur in an amount not greater than 0.005%,
copper in an amount not greater than 0.10%,
aluminum in an amount not greater than 0.01%,
arsenic in an amount not greater than 0.01%,
tin in an amount not greater than 0.01%,
antimony in an amount not greater than 0.003%, and
the balance being iron and unavoidable impurities, and containing a metallic structure having an austenitic grain size number in a range of from 3 to 6.
In the fourth low-alloy heat-resistant steel, cobalt and at least one of niobium, tantalum, nitrogen, and boron are added into the first low-alloy heat-resistant steel. Thereby, the toughness and the unnotched creep properties thereof are further improved. In addition, similar to the first low-alloy heat-resistant steel, the creep properties, in particular, the creep embrittlement resistance, is improved by minimizing a permissible amount of minor impurity elements, such as P, S, Cu, Al, As, Ti, Sb, and the like, which are harmful in causing creep embrittlement, and adjusting an austenitic grain size number of the crystal grain into a range of from 3 to 6.
In these low-alloy heat-resistant steels, it is preferable to contain a composite structure which mainly contains the bainite phase and the pro-eutectoid ferrite phase.
According to the low-alloy heat-resistant steels, the toughness and the creep embrittlement resistant of these steels are improved by precipitating a suitable amount of the pro-eutectoid ferrite phase and effectively using it to adjust the average crystal grain size, while the necessary strength is maintained.
In these low-alloy heat-resistant steels, it is preferable for the pro-eutectoid ferrite phase to be contained in a range of from 5 to 40% by volume.
Since the pro-eutectoid ferrite phase which is usually precipitated in ordinary low-alloy heat-resistant steels is soft, when a large amount of the pro-eutectoid ferrite phase is precipitated, it is difficult to maintain the strength at an early age (0.2% yield strength) and the creep strength high. In addition, since the toughness of the pro-eutectoid ferrite phase is smaller than that of the bainite phase which is an aggregate comprising fine needle structures, when a large amount of the pro-eutectoid ferrite phase is precipitated, the toughness of the material also decreases. Therefore, it has been believed that the pro-eutectoid ferrite phase should, as much as possible not be precipitated. In contrast, in the present invention, after the pro-eutectoid ferrite phase is reinforced by using the matrix reinforcing elements and a suitable amount of the pro-eutectoid ferrite phase is precipitated, the pro-eutectoid ferrite phase is used effectively to reduce the sizes of the crystal grain. Thereby, the toughness and the creep embrittlement resistance are improved. From this point of view, a suitable amount of the pro-eutectoid ferrite phase to be precipitated is specified in the above range.
In addition, in these low-alloy heat-resistant steels, it is preferable for the pro-eutectoid ferrite phase to contain a metallic structure in which carbonitride phases are finely dispersed.
According to these low-alloy heat-resistant steels, it is possible to reinforce the pro-eutectoid ferrite phase and to increase the creep strength of the pro-eutectoid ferrite phase to a level of the creep strength of the bainite phase. Therefore, these low-alloy heat-resistant steels have excellent low temperatures and high temperatures properties. In particular, these low-alloy heat-resistant steels can be used to make large parts.
In other words, the low-alloy heat-resistant steels of the present invention can be easily manufactured. In particular, in the low-alloy heat-resistant steels of the present invention, the quenching from 1,000xc2x0 C. or greater can be proceeded. Due to this, the metallic structure in the center part thereof equals that in the surface layer. Therefore, the low-alloy heat-resistant steels of the present invention have yield strength and toughness, which equal or greater than those of the conventional CrMoV steels, and excellent high temperature properties. In particular, creep embrittlement does not occur in the low-alloy heat-resistant steels of the present invention. Therefore, the low-alloy heat-resistant steels of the present invention are suitable for the materials of turbine rotors.
In order to achieve the objects, the present invention provides a first heat treatment method for a low-alloy heat-resistant steel, comprising the steps of:
heating a steel ingot in a range of from 1,000 to 1,100xc2x0 C., which comprises carbon in an amount of 0.20 to 0.35%, silicon in an amount of 0.005 to 0.35%, manganese in an amount of 0.05 to 1.0%, nickel in an amount of 0.05 to 0.3%, chromium in an amount of 0.8 to 2.5%, molybdenum in an amount of 0.1 to 1.5%, tungsten in an amount of 0.1 to 2.5%, vanadium in an amount of 0.05 to 0.3%, and the balance being iron and unavoidable impurities;
cooling the steel ingot to a certain temperature in a range of from 900 to 700xc2x0 C. by spray-quenching and/or air-blast quenching,
air cooling for from 5 minutes to 5 hours,
cooling again by at least one method consisting of spray-quenching, air-blast quenching, and oil quenching.
In the first heat treatment method, the steel ingot at high temperatures is rapidly cooled and quenched, and after that, the steel ingot is left in the air and is thereby air cooled. Due to this, the cooling rate in the surface layer decreases, and temperature difference between in the surface layer and in the center part decreases. As a result, a suitable amount of the pro-eutectoid ferrite phase is precipitated in the surface layer.
In order to achieve the objects, the present invention provides a second heat treatment method for a low-alloy heat-resistant steel comprising the steps of:
heating a steel ingot in a range of from 1,000 to 1,100xc2x0 C., which comprises carbon in an amount of 0.20 to 0.35%, silicon in an amount of 0.005 to 0.35%, manganese in an amount of 0.05 to 1.0%, nickel in an amount of 0.05 to 0.3%, chromium in an amount of 0.8 to 2.5%, molybdenum in an amount of 0.1 to 1.5%, tungsten in an amount of 0.1 to 2.5%, vanadium in an amount of 0.05 to 0.3%, and the balance being iron and unavoidable impurities;
cooling the steel ingot to a certain temperature in a range of from 800 to 600xc2x0 C. with an average cooling rate of 2xc2x0 C./min or less; and
cooling to 300xc2x0 C. with an average cooling rate in a range of from 2 to 15xc2x0 C./min.
In the second heat treatment method, the steel ingot at high temperatures is cooled at a relatively low cooling rate. During cooling, the pro-eutectoid ferrite phase is precipitated in not only the center part but also in the surface layer. After that, the steel ingot is quenched by increasing the cooling rate.
In the heat treatment methods, it is preferable for the steel ingot to further comprise at least one of niobium in an amount of 0.01 to 0.15%, tantalum in an amount of 0.01 to 0.15%, cobalt in an amount of 0.1 to 3.5%, nitrogen in an amount of 0.001 to 0.05%, and boron in an amount of 0.001 to 0.015%.
According to the heat treatment methods, since such minor elements are added, the crystal grain is fine, and the creep strength thereof is improved.
In addition, in the heat treatment methods, it is preferable that phosphorus be contained in an amount not greater than 0.012%, sulfur be contained in an amount not greater than 0.005%, copper be contained in an amount not greater than 0.10%, aluminum be contained in an amount not greater than 0.01%, arsenic be contained in an amount not greater than 0.01%, tin be contained in an amount not greater than 0.01%, and antimony be contained in an amount not greater than 0.003%, which are impurities contained in the steel ingot.
According to the heat treatment methods, since the allowable range of these minor impurities is limited as above, creep embrittlement can be avoided.
In addition, according to the heat treatment methods of the present invention, even when the material is large, the metallic structure in the surface layer and in the center part of the material can be uniformly by adjusting the cooling rate. The adjustment of the cooling rate is easily carried out. In addition, it is possible to adjust the crystal grain size number into a range of from 3 to 6. Therefore, according to the heat treatment method of the present invention, it is possible to provide materials, which have sufficient yield strength for large elements, high toughness, and excellent high temperature properties, in particular, high creep embrittlement resistant, in a short manufacturing period with low cost.
In addition, in order to achieve the objects, the present invention provides a turbine rotor comprising the low-alloy heat-resistant steel.
According to the turbine rotor, even when the turbine rotor is large, the turbine rotor has relatively uniform high temperature properties throughout the entire turbine rotor. Therefore, the turbine rotor can be used at high temperatures which are higher than conventional service temperatures. Due to this, it is possible to provide an electric power plant having a high energy efficiency. In addition, since the heat treatment, which is used for the turbine rotor, is simple, the manufacturing cost for turbine rotors can be reduced. Furthermore, the turbine rotors of the present invention are effective for reducing the cost required to generate electric power.